Brook troutSalvelinus fontinalis, mottled sculpinsCottus bairdi, and slimy sculpinsCottus cognatusoccur in many Pennsylvania streams that have depressed pH and elevated aluminum concentrations during episodes of high stream discharge (acidic episodes). We performed 20-d in situ cage exposures with these species to determine their relative sensitivities to field conditions. We also exposed fish in the laboratory to synthetic soft water, without added Al, to elevate possible effects of Al on sodium flux rates and pH toxicity. Exposures were in five streams: Two with high pH (>5.60) and low Al concentrations (<80 μg/L) and three with low pH (usually between 5.0 and 5.5) and high Al levels (124–294 μg/L). Exposures were during two low-discharge fall periods, when pH tends to be seasonally higher and Al concentrations lower, and two relatively high-discharge spring seasons, when lower pH and higher Al concentrations are typical. Few fish died (generally < 10%) in the two streams that had higher pH and lower Al concentrations, whereas mortalities typically exceeded 20% and were as high as 100% during spring exposures in the streams with lower pH and elevated Al concentrations. All three species had higher mortality rates in spring, 20–100%, than in fall, 0–29%. Mottled sculpins and slimy sculpins had similar mortality rates and both had lower mortality rates than brook trout when exposed to similar conditions. We compared Na flux rates of mottled sculpins to those of brook trout in laboratory exposures at pH 4.0, 5.0, and 6.0 to determine if low pH alone could account for mortality rates in the field. Because Na flux rates at pH 5.0, without added Al, were similar to flux rates of the controls for both species, high Al levels were believed to have contributed to the increased mortality observed in streams.